Conventionally, although a disc brake device mounted on a vehicle generates braking force by pressing a brake pad against a sliding surface of a disc rotor rotating together with a wheel, the lining of the brake pad is abraded (thinned) by repetitive braking operations. Therefore, there is known a brake pad abrasion detector for informing a user that the brake pad (lining) has reached the application limit thereof. For example, there is a back plate supporting a lining has a sensor piece formed by bending a metal plate and fixed thereto. There is a mechanical abrasion detector that generates a warning sound when a distal end part of the sensor piece comes in contact with the disc rotor and vibrates when the lining is abraded (thinned) to the application limit (Patent Literature 1).